📝 document file sink policy state methods

docs/api/file-sink-policy-method.md

@@ -0,0 +1,74 @@
+---
+name: file-sink-policy-method
+group: api
+category: sink
+update-time: 20260613
+description: Read the current runtime file policy from a FileSink.
+key-word:
+    - file
+    - sink
+    - policy
+    - public
+---
+
+## File-sink-policy-method
+
+Read the current runtime file policy from a `FileSink`. This helper exposes the active append, auto-flush, and rotation settings as one policy object on the concrete sink.
+
+### Interface
+
+```moonbit
+pub fn FileSink::policy(self : FileSink) -> FileSinkPolicy {
+```
+
+#### input
+
+- `self : FileSink` - File sink whose current file policy should be inspected.
+
+#### output
+
+- `FileSinkPolicy` - Current runtime file policy.
+
+### Explanation
+
+Detailed rules explaining key parameters and behaviors
+
+- The returned policy is constructed from the sink's current append, auto-flush, and rotation settings.
+- This helper is broader than `append_mode()` or `auto_flush_enabled()` because it returns the whole policy object.
+- It is observation-only and does not mutate sink state.
+
+### How to Use
+
+Here are some specific examples provided.
+
+#### When Need Full Runtime Policy Visibility
+
+When diagnostics should inspect the active file policy as one object:
+```moonbit
+let policy = sink.policy()
+```
+
+In this example, append, flush, and rotation settings are read together from the direct sink.
+
+#### When Compare Current And Default Policy
+
+When runtime drift from defaults should be inspected explicitly:
+```moonbit
+let current = sink.policy()
+let defaults = sink.default_policy()
+```
+
+In this example, callers can compare current runtime settings with the initial policy snapshot.
+
+### Error Case
+
+e.g.:
+- If callers only need one field from the policy, a narrower helper may be simpler.
+
+- This helper describes policy state, not whether the sink is currently available.
+
+### Notes
+
+1. Use this helper when file policy should be handled as one object.
+
+2. Pair it with `set_policy(...)` for roundtrip-style policy management.